We weren't planning on moving #1 to her big girl bed until much later, but I was talking with a friend who has 2u2 and she said that she transitioned #1 to the bed at 18 months (about 3 months before #2 was born). Supposedly, it made the transition easier?
We're not planning on using the crib for #2 (DD has convertible furniture), so that's not an issue.
I read that Weissbluth says your LO should stay in the crib as long as possible, until 3, because that's when they actually start to understand boundaries (and will stay in their own bed at night).
What are you doing/what did you do?
Re: Transition to toddler bed?
2nd crib here too.
We transitioned at 28 months and only because the older toddler was trying to climb out.
I can tell you that knowing your older one is contained and safe while you deal with the demands of an infant is priceless.
Best $$ we've ever spent.
Total score: 6 pregnancies, 5 losses, 2 amazing blessings that I'm thankful for every single day.